The present invention relates to the transmission of data in a telecommunications network. The present invention more particularly relates to identifying and locating points of failure in a telecommunications network.
Asynchronous transfer mode (ATM) is a standard protocol for transmitting telecommunications data across a telecommunications network. It is based on the transmission of data in fixed size data packets known as ATM cells, wherein each ATM cell has a 48 octet payload portion and a five octet header portion. ATM is well known in the art.
ATM is frequently used for transporting low bit rate data in telecommunications networks, for example, low bit rate voice data. However, standard ATM does not efficiently transport low bit rate data. More specifically, standard ATM tends to add a significant time delay to the transmission of low bit rate data. Unfortunately, low bit rate data, such as low bit rate voice data, is highly sensitive to these data transmission timing delays. Consequently, several different ATM adaptation layers (AALs) have been developed so that ATM can be utilized to transport low bit rate data in a more efficient manner, as is well known in the art.
One of the AALs developed to make ATM more efficient, with respect to low bit rate data, is AAL2. AAL2 is also well known in the art, and it is more thoroughly described in the B-ISDN ATM Adaptation layer Type 2 Specification, ITU Recommendation I.363.2 (herein referred to as "the AAL2 specification"). AAL2 makes ATM a more efficient vehicle for transporting low bit rate data by inserting low bit rate data from any number of different sources into AAL2 data packets, and then multiplexing the AAL2 data packets onto a single ATM connection.
In accordance with the AAL2 specification, the standard format of an AAL2 data packet is similar to that of a standard ATM cell. For example, an AAL2 data packet also has a header portion and a payload portion. However, the header portion of an AAL2 data packet is 3 octets in length and the payload of an AAL2 data packet can vary from 1 octet to 64 octets.
The header portion of an AAL2 data packet more specifically comprises an 8 bit connection identifier (CID) field, a 6 bit length indicator (LI) field, a 5 bit user-to-user information (UUI) field, and a 5 bit header error control (HEC) code. The CID field defines the AAL2 channel to which the AAL2 packet belongs. According to the AAL2 specification, the AAL2 packet may be associated with one of 248 different AAL2 channels. Accordingly, AAL2 data packets from as many as 248 different AAL2 sources can be multiplexed onto a single ATM connection. The LI field, as the name suggests, defines the length of the payload portion of the AAL2 data packet (e.g., the number of octets in the payload portion). The HEC code is specifically used for detecting errors in the header portion of the AAL2 packet.
The purpose of the UUI field is to convey information about the corresponding AAL2 packet between a source node (i.e., the network node that generates the AAL2 packet), a destination node (i.e., the network node to which the AAL2 data packet is being sent), and possibly, a number of intermediate nodes. As stated, there are 5 bits in the UUI field. Therefore, 32 different binary combinations (i.e., codepoints) are possible. In accordance with the AAL2 specification, two of these codepoints are specifically reserved for identifying the corresponding AAL2 packet as an operation and maintenance (OAM) data packet.
A standard ATM cell header includes a 3 bit field called the payload type indicator (PTI) field. The function of the PTI field in an ATM cell header is similar to the function of the UUI field in the AAL2 packet header. For example, two of the eight possible PTI field codepoints are used to identify the corresponding ATM cell as an OAM cell, particularly, an OAM cell that is employed to perform loopback testing. Loopback testing is an OAM function used to detect broken connections in the network. One of these two codepoints specifically identifies the ATM cell as an end-to-end loopback packet. The other codepoint identifies the ATM cell as a segment loopback packet. An end-to-end loopback packet is used for determining whether a broken connection exists somewhere between a source node and a destination node, wherein one or more intermediate nodes exist between the source and destination nodes. However, the precise location of the break cannot be determined. A segment loopback packet is always forwarded from a source node to an adjacent destination node and then back to the source node, and it is only used for determining whether a broken connection exists between that source node and that adjacent destination node.
In practice, the ATM loopback procedure is very complex. For example, the end-to-end loopback packet procedure does not precisely locate a broken connection between a source node and a non-adjacent destination node. In addition, the segment loopback procedure does not detect nor can it precisely locate a broken connection between a source node and a destination node unless the break specifically occurs between the source node and the adjacent destination node specified by the segment loopback packet. In order to precisely locate a broken connection using ATM, the network operator must periodically test every network link and/or connection individually. Typically, this is accomplished as a background task. Consequently, a significant amount of network time and network resources (e.g., bandwidth) are unnecessarily expended. Therefore, a more simplified and efficient procedure is needed to detect the location of broken connections within a network, and to do so on an "as needed" basis rather than on a periodic basis.